U.S. patent number 9,103,650 [Application Number 12/893,393] was granted by the patent office on 2015-08-11 for optical tomographic image generation method and optical tomographic image generation apparatus.
This patent grant is currently assigned to Canon Kabushiki Kaisha. The grantee listed for this patent is Nobuhito Suehira, Mitsuro Sugita. Invention is credited to Nobuhito Suehira, Mitsuro Sugita.
United States Patent |
9,103,650 |
Suehira , et al. |
August 11, 2015 |
Optical tomographic image generation method and optical tomographic
image generation apparatus
Abstract
Provided is a method for easily improving image quality in an
optical tomographic imaging apparatus. An optical tomographic image
generation method of generating a tomographic image of an object
includes: acquiring a signal; performing Fourier transform; and
obtaining the tomographic image. The optical tomographic image
generation method further includes one of the steps of combining a
plurality of the signals acquired within a predetermined time and
combining a plurality of the signals acquired within a
predetermined time after performed the Fourier transform
thereon.
Inventors: |
Suehira; Nobuhito (Kawasaki,
JP), Sugita; Mitsuro (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Suehira; Nobuhito
Sugita; Mitsuro |
Kawasaki
Tokyo |
N/A
N/A |
JP
JP |
|
|
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
|
Family
ID: |
43530922 |
Appl.
No.: |
12/893,393 |
Filed: |
September 29, 2010 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110096333 A1 |
Apr 28, 2011 |
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Foreign Application Priority Data
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Oct 23, 2009 [JP] |
|
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2009-244696 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B
5/0073 (20130101); G01B 9/02083 (20130101); G01B
9/02091 (20130101); G01B 9/02087 (20130101); A61B
3/102 (20130101); A61B 5/0066 (20130101); G01B
9/02044 (20130101); A61B 5/7257 (20130101); A61B
5/7203 (20130101) |
Current International
Class: |
G01B
9/02 (20060101); A61B 3/10 (20060101); A61B
5/00 (20060101) |
Field of
Search: |
;356/497,479 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2008-039651 |
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Feb 2008 |
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JP |
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2008-142443 |
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Jun 2008 |
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JP |
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2008-237238 |
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Oct 2008 |
|
JP |
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2010-110392 |
|
May 2010 |
|
JP |
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2008/052793 |
|
May 2008 |
|
WO |
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2009-034704 |
|
Mar 2009 |
|
WO |
|
Other References
Grant R. Fowles, Introduction to Modern Optics, 1975, Dover
Publications, 2nd Ed., pp. 68-73. cited by examiner .
Office Action dated Dec. 7, 2011, in Japanese Application No.
2009-244696. cited by applicant .
Search Report for corresponding European Appln. No. 10188408.8
dated Feb. 17, 2011. cited by applicant .
Jan. 18, 2013 Chinese Official Action in Chinese Patent Appln. No.
201010515596.0. cited by applicant.
|
Primary Examiner: Hansen; Jonathan
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
What is claimed is:
1. An optical tomographic image generation method of generating at
least one two-dimensional tomographic image of an eye based on a
plurality of intensity signals of a plurality of lines of a
combined light of (a) a return light from the eye to which a
measurement light has been applied and (b) a reference light that
corresponds to the measurement light, the optical tomographic image
generation method comprising the steps of: acquiring the plurality
of intensity signals using a scanning unit which is arranged so as
to move a measurement light that is applied to a retina of the eye
through a cornea of the eye on the retina in a direction crossing
an optical axis of an optical path of the measurement light;
performing Fourier transformations on the plurality of intensity
signals; averaging, in a state of complex numbers, the plurality of
intensity signals after performing Fourier transformations on the
plurality of intensity signals; converting the averaged plurality
of intensity signals into real number data; and generating the at
least one two-dimensional tomographic image of the eye based on the
real number data, wherein the plurality of intensity signals are
acquired after a coherence gate is adjusted to move toward the
retina of the eye within a predetermined time when acquiring one
two-dimensional tomographic image of the eye, and wherein the
predetermined time is acquired based on at least (a) a scanning
speed of the measurement light scanning the retina by using the
scanning unit and (b) a lateral resolution of the measurement light
scanning the retina by using the scanning unit used for acquiring
the plurality of intensity signals.
2. The optical tomographic image generation method according to
claim 1, wherein the step of averaging is a step of weighted
averaging the plurality of intensity signals.
3. The optical tomographic image generation method according to
claim 1, wherein the step of averaging is a step of adjusting
phases of the plurality of intensity signals after performing
Fourier transformations on the plurality of intensity signals.
4. The optical tomographic image generation method according to
claim 3, wherein the step of adjusting phases is performed so that
an average of the plurality of intensity signals is maximized.
5. The optical tomographic image generation method according to
claim 1, further comprising a step of scanning the measurement
light on the eye, wherein the scanning is performed stepwise.
6. The optical tomographic image generation method according to
claim 1, further comprising the steps of: adjusting a ratio of a
longitudinal length to a lateral length of the two-dimensional
tomographic image of the eye acquired by the step of averaging; and
displaying, on a display unit, the two-dimensional tomographic
image of the eye acquired by the step of adjusting.
7. An optical tomographic image generation apparatus that generates
at least one two-dimensional tomographic image of an eye based on a
plurality of intensity signals of a plurality of lines of a
combined light of (a) a return light from the eye to which a
measurement light has been applied and (b) a reference light that
corresponds to the measurement light, the optical tomographic image
generation apparatus comprising: an acquiring unit that acquires
the plurality of intensity signals using a scanning unit which is
arranged so as to move the measurement light that is applied to the
retina of the eye through the cornea of the eye on the retina in a
direction crossing an optical axis of an optical path of the
measurement light; a performing unit that performs Fourier
transformations on the plurality of intensity signals; an averaging
unit that averages, in a state of complex numbers, the plurality of
intensity signals after performing Fourier transformations on the
plurality of intensity signals; a converting unit that converts the
averaged plurality of intensity signals into real number data; and
a generating unit that generates the at least one two-dimensional
tomographic image of the eye based on the real number data, wherein
the plurality of intensity signals are acquired after a coherence
gate is adjusted to move toward the retina of the eye within a
predetermined time when acquiring one two-dimensional tomographic
image of the eye, and wherein the predetermined time is acquired
based on at least (a) a scanning speed of the measurement light
scanning the retina by using the scanning unit and (b) a lateral
resolution of the measurement light scanning the retina by using
the scanning unit used for acquiring the plurality of intensity
signals.
8. The optical tomographic image generation apparatus according to
claim 7, wherein the scanning is performed stepwise.
9. The optical tomographic image generation apparatus according to
claim 7, further comprising a control unit that controls an imaging
timing, wherein the imaging timing is a combination of a plurality
of intervals.
10. The optical tomographic image generation apparatus according to
claim 7, wherein the averaging is weighted averaging of the
plurality of intensity signals.
11. The optical tomographic image generation apparatus according to
claim 7, further comprising: an adjusting unit that adjusts a ratio
of a longitudinal length to a lateral length of the two-dimensional
tomographic image of the eye acquired by the averaging unit; and a
displaying unit that displays, on a display unit, the
two-dimensional tomographic image of the eye acquired by the
adjusting unit.
12. A non-transitory computer-readable media storing
computer-executable instructions, in order to acquire at least one
two-dimensional tomographic image of an eye based on a plurality of
intensity signals of a plurality of lines of a combined light of
(a) a return light from the eye to which a measurement light has
been applied and (b) a reference light that corresponds to the
measurement light, for performing a method comprising the steps of:
acquiring the plurality of intensity signals using a scanning unit
which is arranged so as to move a measurement light that is applied
to a retina of the eye through a cornea of the eye on the retina in
a direction crossing an optical axis of an optical path of the
measurement light; performing Fourier transformations on the
plurality of intensity signals; averaging, in a state of complex
numbers, the plurality of intensity signals after performing
Fourier transformations on the plurality of intensity signals;
converting the averaged plurality of intensity signals into real
number data; and generating the at least one two-dimensional
tomographic image of the eye based on the real number data, wherein
the plurality of intensity signals are acquired after a coherence
gate is adjusted to move toward the retina of the eye within a
predetermined time when acquiring one two-dimensional tomographic
image of the eye, and wherein the predetermined time is acquired
based on at least (a) a scanning speed of the measurement light
scanning the retina by using the scanning unit and (b) a lateral
resolution of the measurement light scanning the retina by using
the scanning unit used for acquiring the plurality of intensity
signals.
13. An optical tomographic image generation method of generating at
least one two-dimensional tomographic image of an eye based on a
plurality of intensity signals of a plurality of lines of a
combined light of (a) a return light from the eye to which a
measurement light has been applied and (b) a reference light that
corresponds to the measurement light, the optical tomographic image
generation method comprising the steps of: performing Fourier
transformations on the plurality of intensity signals; averaging,
after performing Fourier transformations on the plurality of
intensity signals, the plurality of intensity signals in a state of
complex numbers within a predetermined time in a scanning time of
the measurement light applied to the eye when acquiring one
two-dimensional tomographic image of the eye; converting the
averaged plurality of intensity signals into real number data; and
generating one two-dimensional tomographic image of the eye in the
scanning time based on the real number data.
14. An optical tomographic image generation apparatus for
generating at least one two-dimensional tomographic image of an eye
based on a plurality of intensity signals of a plurality of lines
of a combined light of (a) a return light from the eye to which a
measurement light has been applied and (b) a reference light that
corresponds to the measurement light, the optical tomographic image
generation apparatus comprising: a performing unit that performs
Fourier transformations on the plurality of intensity signals; an
averaging unit that averages, after performing Fourier
transformations on the plurality of intensity signals, the
plurality of intensity signals in a state of complex numbers within
a predetermined time in a scanning time of the measurement light
scanning the eye when acquiring one two-dimensional tomographic
image of the eye; a converting unit that converts the averaged
plurality of intensity signals into real number data; and a
generating unit that generates the at least one two-dimensional
tomographic image of the eye based on the real number data.
15. An optical tomographic image generation method of generating at
least one two-dimensional tomographic image of an eye based on a
plurality of intensity signals of a plurality of lines of a
combined light of (a) a return light from the eye to which a
measurement light has been applied and (b) a reference light that
corresponds to the measurement light, the optical tomographic image
generation method comprising the steps of: performing Fourier
transformations on the plurality of intensity signals; and
averaging the plurality of intensity signals in a state of complex
numbers after performing Fourier transformations on the plurality
of intensity signals; converting the averaged plurality of
intensity signals into real number data; and generating the at
least one two-dimensional tomographic image of the eye based on the
real number data.
16. An optical tomographic image generation apparatus for
generating at least one two-dimensional tomographic image of an eye
based on a plurality of intensity signals of a plurality of lines
of a combined light of (a) a return light from the eye to which a
measurement light has been applied and (b) a reference light that
corresponds to the measurement light, the optical tomographic image
generation apparatus comprising: a performing unit that performs
Fourier transformations on the plurality of intensity signals; an
averaging unit that averages the plurality of intensity signals in
a state of complex numbers after performing Fourier transformations
on the plurality of intensity signals; a converting unit that
converts the averaged plurality of intensity signals into real
number data; and a generating unit that generates the at least one
two-dimensional tomographic image of the eye based on the real
number data.
17. An optical tomographic image generation apparatus for
generating at least one two-dimensional tomographic image of an eye
based on a plurality of intensity signals of a plurality of lines
of a combined light of (a) a return light from the eye to which a
measurement light has been applied through a scanning unit, and (b)
a reference light that corresponds to the measurement light, the
optical tomographic generation apparatus comprising: a performing
unit that performs Fourier transformations on the plurality of
intensity signals; an averaging unit that averages, after
performing Fourier transformations on the plurality of intensity
signals, the plurality of intensity signals in a state of complex
numbers in a range based on a lateral resolution of the measurement
light scanning the eye when acquiring one two-dimensional
tomographic image of the eye by scanning the eye with the
measurement light by the scanning unit; a converting unit that
converts the averaged plurality of intensity signals into real
number data; and a generating unit that generates the at least one
two-dimensional tomographic image of the eye based on the real
number data.
18. The optical tomographic image generation apparatus according to
claim 17, wherein a scanning distance of the scanning unit is equal
to or smaller than a several-fold length of the lateral
resolution.
19. The optical tomographic image generation apparatus according to
claim 17, wherein the lateral resolution corresponds to a size of
the measurement light on the eye.
20. The optical tomographic image generation apparatus according to
claim 17, further comprising: a determining unit that determines a
scanning speed of the measurement light based on the lateral
resolution; and a control unit that controls the scanning unit
based on the determined scanning speed.
21. The optical tomographic image generation apparatus according to
claim 17, wherein the averaging unit weighted averages the
plurality of intensity signals.
22. The optical tomographic image generation apparatus according to
claim 17, further comprising: an adjusting unit that adjusts a
ratio of a longitudinal length to a lateral length of the
two-dimensional tomographic image of the eye acquired by the
averaging unit; and a displaying unit that displays, on a display
unit, the two-dimensional tomographic image of the eye acquired by
the adjusting unit.
23. An optical tomographic image generation method of generating at
least one two-dimensional tomographic image of an eye based on a
plurality of intensity signals of a plurality of lines of a
combined light of (a) a return light from the eye to which a
measurement light has been applied through a scanning unit and (b)
a reference light that corresponds to the measurement light, the
optical tomographic generation method comprising the step of:
performing Fourier transformations on the plurality of intensity
signals; averaging, after performing Fourier transformations on the
plurality of intensity signals, the plurality of intensity signals
of the plurality of lines in a state of complex numbers in a range
based on a lateral resolution of the measurement light scanning the
eye when acquiring one two-dimensional tomographic image of the eye
by scanning the eye with the measurement light by the scanning
unit; converting the averaged plurality of intensity signals into
real number data; and generating the at least one two dimensional
tomographic image of the eye based on the real number data.
24. The optical tomographic image generation method according to
claim 23, wherein a scanning distance of the scanning unit is equal
to or smaller than a several-fold length of the lateral
resolution.
25. The optical tomographic image generation method according to
claim 23, wherein the lateral resolution corresponds to a size of
the measurement light on the eye.
26. The optical tomographic image generation method according to
claim 23, further comprising the steps of: determining a scanning
speed of the measurement light based on the lateral resolution; and
controlling the scanning unit based on the determined scanning
speed.
27. The optical tomographic image generation method according to
claim 23, the step of averaging is a step of weighted averaging the
new intensity signals.
28. The optical tomographic image generation method according to
claim 23, further comprising the steps of: adjusting a ratio of a
longitudinal length to a lateral length of the two-dimensional
tomographic image of the eye acquired by the step of averaging; and
displaying, on a display unit, the two-dimensional tomographic
image of the eye acquired by the step of adjusting.
29. A computer program stored on a non-transitory computer-readable
medium that causes a computer to execute the optical tomographic
image generation method according to claim 23.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical tomographic image
generation method and an optical tomographic image generation
apparatus that are used for generating a tomographic image of an
object.
2. Description of the Related Art
An imaging apparatus using optical coherence tomography (OCT) in
which interference of low coherent light is utilized (hereinafter,
also referred to as OCT apparatus) is now put into practical use.
This imaging apparatus is capable of acquiring a tomographic image
at a depth resolution of several micrometers, which leads to
high-resolution imaging of a tomographic image of an object.
Japanese Patent Application Laid-Open No. 2008-237238 discloses an
optical image measurement device that is intended to improve image
quality of an image to be formed. This device forms multiple
tomographic images of an eye fundus and stores the formed images.
Then, one of the tomographic images and a tomographic image
adjacent thereto are used for an arithmetic operation, to thereby
form a new tomographic image. As a result, the image quality of the
formed image can be improved.
In Japanese Patent Application Laid-Open No. 2008-237238 described
above, the image quality is improved to some extent, but is not
improved sufficiently. Particularly in a case where the object
moves, its position varies between the tomographic images, which
raises a risk that signal waveforms are rounded.
Therefore, further improvement in image quality is demanded.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above-mentioned
problem, and it is therefore an object of the present invention to
achieve further improvement in image quality. Another object of the
present invention is to achieve improvement in image quality even
when an object moves, by suppressing influence of the movement of
the object.
According to the present invention, there is provided an optical
tomographic image generation method of generating a tomographic
image of an object, including the steps of: acquiring a signal;
performing Fourier transform; and obtaining a tomographic image, in
which the optical tomographic image generation method further
includes one of the steps of combining a plurality of the signals
acquired within a predetermined time and combining a plurality of
the signals acquired within a predetermined time after performing
the Fourier transform thereon.
According to the present invention, there is also provided an
optical tomographic image generation apparatus for splitting a
light from a light source into a measurement light and a reference
light, guiding the measurement light to an object through a
measurement light path, and guiding the reference light to a
reference mirror through a reference light path, to thereby
generate a tomographic image of the object by using a return light,
which is the measurement light reflected or scattered by the
object, the reference light reflected on the reference mirror, and
a combined light obtained by combining the return light and the
reference light, the optical tomographic image generation apparatus
including a unit for scanning the object and a unit for controlling
imaging timing and further includes a unit for combining signals
acquired within a predetermined time.
According to the present invention, improvement in image quality is
achieved by combining the plurality of the signals acquired within
the predetermined time or combining signals obtained after
performing the Fourier transform on the plurality of the
signals.
Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference
to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory diagram illustrating signal processing
steps according to a first embodiment of the present invention;
FIG. 2 is an explanatory diagram illustrating a Michelson OCT
apparatus according to the first embodiment of the present
invention;
FIGS. 3A, 3B, 3C, and 3D are explanatory charts illustrating a
relation between a position of a scanner with respect to a lapse of
time and measurement timing thereof according to the first
embodiment of the present invention;
FIG. 4 is an explanatory diagram illustrating a Mach-Zehnder OCT
apparatus according to a second embodiment of the present
invention;
FIGS. 5A and 5B are explanatory diagrams illustrating signal
processing steps according to the second embodiment of the present
invention; and
FIG. 6 is an explanatory diagram illustrating signal processing
steps according to a third embodiment of the present invention.
DESCRIPTION OF THE EMBODIMENTS
Preferred embodiments of the present invention will now be
described in detail in accordance with the accompanying
drawings.
According to the present invention, there is provided an optical
tomographic image generation method of generating a tomographic
image of an object, including the steps of: acquiring a signal;
performing Fourier transform; and obtaining the tomographic image,
in which the optical tomographic image generation method further
includes one of the steps of combining a plurality of the signals
acquired within a predetermined time and combining a plurality of
the signals acquired within a predetermined time after performing
the Fourier transform thereon. FIG. 1 illustrates a case where the
optical tomographic image generation method includes the step of
combining a plurality of the signals acquired within a
predetermined time.
EMBODIMENTS
First Embodiment
Next, a first embodiment of the present invention is described. In
this embodiment, an imaging apparatus using a Michelson
interferometer is employed for generating a tomographic image, but
the imaging apparatus employable for the present invention is not
limited thereto. Further, signal processing of this embodiment is
characterized in combining multiple signals after the signals are
acquired.
Michelson Interferometer
An optical coherence tomographic imaging apparatus (hereinafter,
also referred to as OCT apparatus) according to the first
embodiment is described with reference to FIG. 2. FIG. 2 is a
schematic diagram illustrating the imaging apparatus using the
Michelson optical system (Michelson interferometer) according to
this embodiment.
A light emitted from a light source 201 passes through a fiber 202
and a lens 203-1, and is split into a measurement light 214 and a
reference light 213 by a beam splitter 204. The measurement light
214 passes through an XY scanner 208, an objective lens 205-1, and
an objective lens 205-2, and enters an eye 217, which is an object.
Then, the measurement light that has entered the eye passes through
a cornea 216, and reaches a retina 218.
A return light 215 reflected and scattered on the retina 218 of the
eye 217 returns by passing through the objective lens 205-2, the
objective lens 205-1, the XY scanner 208, and the beam splitter 204
in the stated order. Further, the return light 215 is guided to a
spectrometer 211 through a lens 203-2. The spectrometer 211
includes a lens, a grating, and an image-pickup device. Used as the
image-pickup device is a line sensor of a charge coupled device
(CCD) type or a complementary metal oxide semiconductor (CMOS)
type. Signals obtained by the line sensor of the spectrometer 211
are sent to a computer 212 and stored in a memory. The stored
signals are then subjected to processing described later.
On the other hand, the reference light 213 passes through a
dispersion compensation glass 207, is reflected on a reference
mirror 209, passes again through the dispersion compensation glass
207, and returns to the beam splitter 204. The dispersion
compensation glass 207 is used for compensating for dispersion
caused in the eye 217 and the objective lenses 205-1 and 205-2. The
reference mirror 209 can adjust an optical path length of a
reference light path by means of a mirror adjustment mechanism 210.
These reference light 213 and return light 215 are combined by the
beam splitter 204. Then, the combined light is guided to the
spectrometer 211. Note that a portion on a measurement light path
having an optical path length matching with that of the reference
light path is referred to as a coherence gate. In a case where the
retina 218 of the eye 217 is measured, the position of the
reference mirror 209 is adjusted so that the coherence gate becomes
close to the retina 218.
As the light source 201, a super luminescent diode (SLD), which is
a typical low coherent light source, is used. As to the wavelength
thereof, for example, the center wavelength is 840 nm and the
bandwidth is 50 nm. Note that, the bandwidth is an important
parameter because of its influence on the resolution of a
tomographic image to be obtained in an optical axis direction.
Besides, the type of the selected light source 201 is the SLD
herein, and alternatively, amplified spontaneous emission (ASE) may
be used therefor as long as the light source 201 emits low coherent
light. Obviously, depending on contents of the object, another
light source such as a halogen lamp may be used. Note that, the
wavelength influences resolution of the tomographic image to be
obtained in a lateral direction, and hence a short wavelength is
desired when the resolution in the lateral direction is
important.
The computer 212 performs arithmetic processing and control that
are described later, and also controls the spectrometer 211, the XY
scanner 208, the mirror adjustment mechanism 210, and a focus
adjustment mechanism 206. Obviously, the computer 212 is also
capable of data input, image processing, image display, and data
storage.
Signal Processing Steps
The signal processing executed by the OCT apparatus illustrated in
FIG. 2 is described with reference to FIG. 1. Among Steps A1 to A7,
a combination step M1 is provided between Steps A2 and A3.
In Step A1, measurement is started. At this stage, the OCT
apparatus is activated and an eye as an object is set for the
measurement. Additionally, adjustment necessary for the measurement
is performed by an operator so that the measurement can be
started.
In Step A2, signals are acquired. In the following, a case of
acquiring one two-dimensional tomographic image is described as an
example. The XY scanner 208 is moved in an X direction of FIG. 2,
which is perpendicular to the optical axis of the eye 217. FIGS. 3A
to 3D are charts illustrating a relation between a position of the
XY scanner 208 with respect to a lapse of time and imaging timing
thereof. FIG. 3A illustrates a case where the XY scanner 208 moves
in the X direction stepwise. The number of steps is, for example,
512. FIG. 3B illustrates measurement timings, and illustrates
control of imaging performed three times at regular intervals in
one step. Measurement intervals 301 are equal both for one step and
between steps. Hence, the imaging is performed 1,536 (=512.times.3)
times. Note that, as to data acquired per imaging, a
one-dimensional array of 1,024 elements is obtained in a case of a
line sensor having 1,024 pixels.
Therefore, since there are 1,536 lines, a two-dimensional array of
1,024.times.1,536 elements is obtained finally. Note that, the data
of each line is stored as data of each column of the
two-dimensional array.
Note that, the XY scanner 208 may be moved in the X direction
successively as illustrated in FIG. 3C. The successive movement
reduces loads on the scanner and leads to smooth movement as
compared with the above-mentioned case. The measurement intervals
may be equal, and alternatively, such different intervals as a
measurement interval 302 and a measurement interval 303 as
illustrated in FIG. 3D may be provided. In this case, regular
intervals are provided among three successive imaging operations
for obtaining signals to be combined, while a different measurement
interval is provided between adjacent two sets of the imaging
operations. Obviously, the combination of the position control and
the imaging timing is freely determined. The position control of
the successive movement illustrated in FIG. 3C and the regular
imaging timing illustrated in FIG. 3B may be employed in
combination.
Note that, in a case of combining signals acquired through imaging
performed while moving the XY scanner 208 in the X direction
successively, when the moving distance is large, signal components
may be rounded (data to be combined counteract each other so that
original signal components may be lost). Therefore, it is desired
that a moving distance 304 be equal to or smaller than a
several-fold length of the lateral resolution of the OCT apparatus
(generally determined based on a beam diameter of the measurement
light on the object). Specifically, the moving distance 304 ranges
from several micrometers to several hundred micrometers. Note that,
in a case where a moving speed of the scanner is known, the
predetermined distance may be converted into a time, and the time
thus obtained may be set as a predetermined time. As long as the
condition of the predetermined distance or the predetermined time
is satisfied, the data may be acquired by rotating the XY scanner
208 like circle scanning instead of linear movement in the X or Y
direction.
For example, in a case where imaging is performed by a 20-kHz line
scanner for a range of 6 mm, the moving speed of the scanner at the
time of scanning the object is 78 (=6/1,536.times.20 k) mm/s. The
moving distance when averaged for three lines is 12
(=78.times.(3/20 k)) .mu.m. When the lateral resolution of the OCT
apparatus is 20 .mu.m, the moving distance falls within a range in
which signal loss is less likely to occur even after the
combination. Note that, the time necessary to acquire an image of
512 lines at 20 kHz is 25.6 (=512/20 k) msec. In a case of
acquiring three images, at least 76.8 (=25.6.times.3) msec is
necessary. The time necessary to acquire signals of three lines is
0.15 (=3/20 k) msec, in which the object does not move so
widely.
In Step M1, the signals are combined. In this step, one-dimensional
arrays of three adjacent spectra are averaged and a new
one-dimensional array is created. Specifically, in the
1,024.times.1,536 two-dimensional array, one-dimensional arrays of
3j-th, (3j+1)th, and (3j+2)th columns (j is an integer ranging from
0 to 511) are averaged. As a result, a 1,024.times.512
two-dimensional array is obtained. Through the averaging, noise
components may be removed.
As compared with a case of averaging performed in a process after
Step A3, calculation may be performed with a smaller number of
data, which leads to an effect of shortening the calculation time.
Note that, the averaging may be weighted averaging, but the noise
removal effect may vary when the weights are different. Note that,
the number of lines to be averaged is not necessarily three in the
case of the combination of control illustrated in FIGS. 3C and 3B,
and may be an arbitrary integer that satisfies the condition of the
lateral resolution described above. Further, data that is
considered to have a measurement error may be removed.
In Step A3, wavelength-wavenumber conversion is performed. In
general, data from the spectrometer 211 includes a wavelength and
the intensity at the wavelength. Further, sampling is performed at
regular intervals with regard to the wavelength. First, a function
of intensity data with regard to the wavelength is created. Then,
each wavelength is converted into a wavenumber, and a function of
intensity data with regard to the wavenumber is created. The
wavenumber is an inverse of the wavelength, and hence 1,024
wavenumbers are allocated at regular intervals. Then, intensity
data corresponding to the wavenumbers is calculated. The
calculation method to be employed is, for example, interpolation,
which may be general linear interpolation or spline interpolation.
In this case, a linear arithmetic operation is desired. As a
result, a two-dimensional array of 1,024.times.512 elements
including intensities at regular intervals with regard to the
wavenumber is obtained.
In a case where the spectrometer 211 can perform sampling at
regular intervals with regard to the wavenumber, this step may
obviously be omitted as long as an error due to the
wavelength-wavenumber conversion is negligible.
In Step A4, Fourier transform is performed. In this step, the
intensity values arranged at regular intervals with regard to the
wavenumber are subjected to discrete Fourier transform for each
column. As a result, a 1,024.times.512 two-dimensional array of
complex numbers is obtained. Note that, an m-th row and a
(1,024-m)th row of each column have the same intensity value
because of the characteristics of the Fourier transform. Therefore,
0th to 511th rows are extracted and a 512.times.512 two-dimensional
array of complex numbers is obtained.
In Step A5, the complex number data is converted into real number
data. It is not the linear arithmetic operation that is used for
converting the complex number into a real number. Therefore,
averaging before Step A5 is fundamentally different from averaging
after Step A5. The difference is described in a second
embodiment.
In Step A6, a tomographic image is obtained. In this step, a range
is further adjusted in the 512.times.512 two-dimensional array.
The range to be adjusted is, for example, the ratio of a
longitudinal length to a lateral length. To adjust the range, the
number of pixels is increased and decreased through interpolation.
Further, contrast is adjusted. The adjustment of the contrast
refers to general correction of the .gamma. value used for image
processing. As a result, an image suitable for doctor's diagnosis
is obtained. Then, the obtained tomographic image is displayed on a
display screen of the computer 212.
In Step A7, the process ends. The process starting from the
measurement using the OCT apparatus to the image display has been
described above, and alternatively, for example, the
above-mentioned process may be applied to data of multiple frames
acquired via a network, to thereby obtain an image with reduced
noise from a single tomographic image.
According to this embodiment, a high-quality image can be obtained
by combining signals acquired within a predetermined time.
Second Embodiment
Next, a second embodiment of the present invention is described. In
this embodiment, an imaging apparatus using a Mach-Zehnder
interferometer is employed for generating a tomographic image, but
the imaging apparatus employable for the present invention is not
limited thereto. Further, signal processing of this embodiment is
characterized in signal combination performed after Fourier
transform.
Mach-Zehnder Interferometer
An optical coherence tomographic imaging apparatus according to the
second embodiment is described with reference to FIG. 4. FIG. 4 is
a schematic diagram illustrating the imaging apparatus using the
Mach-Zehnder optical system according to this embodiment.
Differences from the first embodiment are described below.
A light emitted from the light source 201 passes through a fiber
coupler 401-1, and is split into a measurement light 214 and the
reference light 213.
The measurement light enters a port 1 of a circulator 402-2, exits
from a port 2 thereof, and reaches a lens 403-2. Further, the
measurement light passes through a XY scanner 208, an objective
lens 205-1, an objective lens 205-2, and a cornea 216 of an eye
217, and reaches a retina 218. The return light 215 scattered and
reflected on the retina returns by passing through the objective
lens 205-2, the objective lens 205-1, the XY scanner 208, and the
lens 403-2, enters the port 2 of the circulator 402-2, exits from a
port 3 thereof, and reaches a fiber coupler 401-2.
On the other hand, the reference light 213 enters a port 1 of a
circulator 402-1, exits from a port 2 thereof, passes through a
lens 403-1 and a dispersion compensation glass 207, and is
reflected on a reference mirror 209. The reflected reference light
213 returns to the port 2 of the circulator 402-1 by passing
through the dispersion compensation glass 207 and the lens 403-1,
exits from a port 3 of the circulator 402-1, and reaches the fiber
coupler 401-2. The reference mirror 209 can adjust the optical path
length by means of the mirror adjustment mechanism 210. The
reference light 213 and the return light 215 are combined by the
fiber coupler 401-2, and the combined light is guided to the
spectrometer.
Signal Processing Steps
The signal processing executed by the OCT apparatus illustrated in
FIG. 4 is described with reference to FIGS. 5A and 5B. FIG. 5A
illustrates a case where, among Steps A1 to A7, a combination step
M2 is provided between Steps A4 and A5. FIG. 5B illustrates a case
where a combination step M3 is provided between Steps A5 and
A6.
The difference between the two cases is now described. First, in
the case of Step M2, complex number data obtained in Step A4 is
combined, and the resultant complex number data is passed to Step
A5. In the combination step M2, simple averaging or weighted
averaging is performed on the complex number data. On the other
hand, in the case of Step M3, real number data calculated in Step
A5 is combined, and the resultant real number data is passed to
Step A6. In the combination step M3, simple averaging or weighted
averaging is performed on the real number data.
The combination based on the complex number in Step M2 is
fundamentally different from the combination based on the real
number in Step M3. The difference is described with reference to
the following mathematical expressions. In the following, an
element 1 and an element 2 of the complex numbers are represented
by Expression 1-1 and Expression 1-2 using an imaginary unit i,
respectively. a.sub.0+b.sub.0i=r.sub.0e(i.phi..sub.0) Expression
1-1 a.sub.1+b.sub.1i=r.sub.1e(i.phi..sub.1) Expression 1-2
When the complex numbers are added together and then the resultant
complex number is converted into a real number, data represented by
Expression 2 is obtained. {square root over
((a.sub.0+a.sub.1).sup.2+(b.sub.0+b.sub.1).sup.2)}{square root over
((a.sub.0+a.sub.1).sup.2+(b.sub.0+b.sub.1).sup.2)} Expression 2
When the complex numbers are converted into real numbers and then
the resultant real numbers are added together, data represented by
Expression 3 is obtained. {square root over
(a.sub.0.sup.2+b.sub.0.sup.2)}+ {square root over
(a.sub.1.sup.2+b.sub.1.sup.2)} Expression 3
When Expression 2 and Expression 3 are squared and then common
parts are subtracted, a relation of Expression 4 is established
(proof is easily obtained by further squaring both sides).
a.sub.0a.sub.1+b.sub.0b.sub.1.ltoreq. {square root over
(a.sub.0.sup.2+b.sub.0.sup.2)} {square root over
(a.sub.1.sup.2+b.sub.1.sup.2)} Expression 4
In other words, the value of Expression 2 is equal to or smaller
than the value of Expression 3. This fact is important in the noise
removal method. Specifically, in a case of random noise, a plus
component and a minus component exist. When the two components are
added together in the state of the complex number, those components
are canceled. Therefore, in theory, when the steps of FIGS. 5A and
5B are compared with each other, noise is further reduced through
the steps of FIG. 5A. In contrast, when real numbers are obtained
and then added together, the cancellation effect is limited. Note
that, in the first embodiment, the linear arithmetic operation is
performed until the Fourier transform, and hence the same effect
may be obtained as in the case of adding the components together in
the state of the complex number. Note that, the simple averaging is
more desired than the weighted averaging because the noise is
equivalent.
Table 1 shows comparison in signal-to-noise ratio (SNR) between the
processing involving the steps of FIG. 1 and the processing
involving the steps of FIG. 5B. The unit is decibel. As the
position control for the scanner, the case of FIG. 3C where the
position of the scanner is shifted successively is applied. As the
imaging timing, the case of FIG. 3B where sampling is performed at
regular intervals is applied. The imaging object is centered on a
macula of a normal eye, and a retina is measured in the range of
approximately 6 mm. The number of lines is 2,048. The same original
data is used and values of the SNR are obtained through each
processing in cases where: (1) every four lines are extracted from
2,048 lines, to thereby create a tomographic image of 512 lines,
which is not subjected to combination; (2) every two lines are
extracted from 2,048 lines to obtain 1,024 lines, and further, two
adjacent lines are combined, to thereby create a tomographic image
of 512 lines; and (3) four lines of 2,048 lines are combined, to
thereby create a tomographic image of 512 lines. In the case of
combining lines before the complex numbers are converted into real
numbers, the SNR is improved as the number of combined lines
becomes larger. On the other hand, in the case of combining lines
after the complex numbers are converted into real numbers, the SNR
is almost constant. When the case where averaging processing is not
performed is compared with the case where averaging processing is
performed after real numbers are obtained, a smoother image is
obtained in the latter case. Note that, the SNR used in this
comparison refers to a ratio of a maximum value of each pixel to a
minimum value of root mean squares (RMSs) of noise of the rows
thereof.
TABLE-US-00001 TABLE 1 Comparison in SNR Steps of Steps of FIG. 1
FIG. 5B 512 (no averaging) 40.22 40.22 1,024 (two-line averaging)
42.84 40.27 2,048 (four-line averaging) 45.74 40.35
According to this embodiment, a high-quality image can be obtained
by combining signals acquired within a predetermined time.
Third Embodiment
Next, a third embodiment of the present invention is described. In
signal processing of this embodiment, phase adjustment is performed
after the Fourier transform, and then signals are combined.
Signal Processing Steps
The signal processing of this embodiment is described with
reference to FIG. 6. A phase adjustment step and a combination step
are provided between Steps A4 and A5. Differences from the second
embodiment are described below.
In Step P1, complex number data are subjected to phase adjustment.
In this step, phases of arrays of three adjacent lines are
adjusted. First, the data are converted into sets of polar
coordinates. An amplitude component and a phase component can be
obtained from the polar coordinates. With the first line of the
three lines set as the 3j-th column (j is an integer ranging from 0
to 511), the phase of the one-dimensional array of the (3j+1)th
column is adjusted with reference to the phase of the 3j-th column
(or the phase of the (3j+2)th column is adjusted with reference to
a result of combination of the 3j-th column and the (3j+1)th
column). First, new arrays are created by changing a phase
component of the array of the (3j+1)th column. When the phase
component is changed by, for example, 10 degrees from 0 degrees to
350 degrees, 36 different one-dimensional arrays may be
obtained.
In Step M3, combination is performed. The one-dimensional array of
the 3j-th column is combined with the 36 different one-dimensional
arrays, respectively. In this step, simple averaging is performed,
to thereby newly obtain 36 different arrays.
In Step A5, real number data are obtained. There are obtained 36
different arrays of real numbers from the 36 different arrays
obtained through the combination.
In Step P2, selection is performed. From among the 36 different
arrays obtained through the combination, an array in which the
signal is maximum is selected. While the noise removal effect is
mainly obtained in the first and second embodiments, the signal can
be maximized in this embodiment. Further, in the case of random
noise, the noise is random even with the phase adjustment, and
hence the noise removal effect is still obtained.
In Step P3, it is determined whether or not the combination is
finished. Specifically, the three lines in the 3j-th column to the
(3j+2)th column are combined, to thereby create a new 3j-th column.
It is determined whether or not such combination is finished for
all j's. When the combination is finished, the processing proceeds
to Step A6. When the combination is not finished, the processing
returns to Step P1.
According to this embodiment, a high-quality image can be obtained
by using data of adjacent lines.
The above-mentioned optical tomographic image generation methods
according to the present invention may be executed by using a
computer with the procedure for performing the respective steps
implemented by a computer program.
Other Embodiments
Aspects of the present invention can also be realized by a computer
of a system or apparatus (or devices such as a CPU or MPU) that
reads out and executes a program recorded on a memory device to
perform the functions of the above-described embodiment(s), and by
a method, the steps of which are performed by a computer of a
system or apparatus by, for example, reading out and executing a
program recorded on a memory device to perform the functions of the
above-described embodiment(s). For this purpose, the program is
provided to the computer for example via a network or from a
recording medium of various types serving as the memory device
(e.g., computer-readable medium). In such a case, the system or
apparatus, and the recording medium where the program is stored,
are included as being within the scope of the present
invention.
While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is
not limited to the disclosed exemplary embodiments. The scope of
the following claims is to be accorded the broadest interpretation
so as to encompass all such modifications and equivalent structures
and functions.
This application claims the benefit of Japanese Patent Application
No. 2009-244696, filed Oct. 23, 2009, which is hereby incorporated
by reference herein in its entirety.
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